The operating pressure of total isomerization process (TIP) units, since the early 1970's, has typically been an economically dictated compromise of the optimum isomerization reactor and adsorber operating pressures.
The reactors have typically operated most efficiently at high pressures (300-400 psia) where the required hydrogen partial pressure can be met at a reasonably low recycle gas rate and compression ratio. Exemplary of teachings with regard to reactor conditions are those in U.S. Pat. No. 3,475,345 and U.S. Pat. No. 3,527,835 to Benesi, U.S. Pat. No. 3,615,188 to Kouwenhoven et al, and U.S. Pat. No. 3,842,114 to Sie.
Adsorber units, however, have typically operated more efficiently at lower pressures (200-250 psia) where the moles of stripping gas are more efficient and bed void storages of gas are lower. See in this regard U.S. Pat. No. 4,176,053 to Holcombe which calls for a non-sorbable purge gas, such as nitrogen, hydrogen, helium or methane, and specifically exemplifies a pressure of 250 psia.
Although adsorber and reactor units can perform over a wide range of pressures, to date, adsorption pressures in the range of 280 to 300 psia have been found to be at the optimum TIP compromise pressure for performance-based economics. In other words, this pressure range typically employed in the prior art was believed to economically satisfy both the reactor hydrogen partial pressure and the stripping requirements of the adsorber section.
Because the TIP units consume some hydrogen in the reactor and some is lost in solution and due to venting, makeup hydrogen must be added. Typically, this has been obtained from hydrogen-containing gas streams containing from 60 to 97 mole percent hydrogen and impurities. The higher purity stream, e.g., from steam reforming of methane, are typically more costly than desired and the more usual practice has been to employ catalytic reformer off gas streams which typically contain a major proportion of hydrogen, e.g., from 60 to 85 mole percent, and a minor proportion of light hydrocarbons, e.g., from 15 to 40 mole percent, with other impurities. The use of catalytic reformer off gas has generally been considered clean and favorable to the operation of the total isomerization process. However, the use of other, less pure hydrogen-containing refinery streams has been restricted due to the presence of larger amounts of light hydrocarbons and other impurities such as hydrogen sulfide, water, chlorides, and other impurities.
The predominant impurity in the typical prior art makeup gas streams is methane, along with decreasing amounts of ethane and propane, and butane (light hydrocarbons), and other impurities. The light hydrocarbons from the makeup gas, along with a smaller amount produced by cracking in the reactor, tend to build up in concentration within the recycle gas loop and adversely impact the selection of the TIP operating pressures and recycle gas rate. The other impurities, e.g., hydrogen sulfide, water, and chlorides, can be maintained at acceptable levels in the recycle by venting a small proportion of the stream as is necessary to control levels of total impurities in the recycle.
In Canadian Pat. No. 1,064,056, Reber et al describe a process for hydrocarbon separation and isomerization wherein large fluctuations in the concentration of either n-pentane or n-hexane in the reactor feed are prevented by suitably controlling the operation of a three bed adsorber system. According to the disclosure, no more than two beds are being desorbed at any given time and the terminal stage of desorption in one of the three beds is contemporaneous with the initial stage of desorption in another of the three beds.
The Canadian disclosure states the the hydrogen stream used as a purge gas in desorbing the adsorption bed and as the hydrogenation material in the isomerization reactor, need not be pure and is generally composed of one or a combination of two or more refinery hydrogen streams such as a reformer hydrogen and the like. It is disclosed that any impurities should be relatively non-sorbable and innert toward the adsorbent, catalyst and hydrocarbon. It is also disclosed that light hydrocarbons are produced in the catalytic unit, and that the recycle hydrogen stream is preferably at least 60 mole percent hydrogen.
The processes specifically exemplified by the Canadian patent employ "adsorber lead" flow wherein the fresh feed is first passed through the adsorbers to remove non-normal hydrocarbons prior to entering the reactor. This requires adsorbers of significantly increased size and larger recycle inventories. The exemplified reactor pressures of 220 psia require high hydrogen concentrations in the reactor to achieve a hydrogen partial pressure sufficient to protect the catalyst, and this in turn forces operation at lower than desired hydrocarbon reactant concentrations. The effluent from the reactor in Example 1 is disclosed as containing about 68 mole percent hydrogen and the feed in Example 2 is disclosed as containing about 60 mole percent hydrogen. With total pressure within the reactor exemplified at 220 psia, these high hydrogen concentrations and the presence of high quantities of light hydrocarbons in the recycle to the reactor leave room for only lower than desired concentrations of hydrocarbon reactants. Thus, while the Canadian process does employ low reactor total pressures, no gains due to decreased recycle rates are achieved, and the size of the adsorbers is increased. Moreover, these lower total pressures, combined with a lower concentration of reactants, results in a decreased reactant residence time within the reactor.
In U.S. Pat. No. 4,210,771, Holcombe describes a total isomerization process which reduces the recycle rate to the reactor while still maintaining a sufficient hydrogen partial pressure to protect the catalyst against coking. The partial pressure of the hydrogen in the reactor is a function of the hydrogen concentration. As with the earlier prior art, Holcombe's specifically disclosed recycle stream contains at least 50 mole percent hydrogen as well as desorbed normal paraffins and relatively non-sorbable light hydrocarbons. Some hydrogen is consummed during processing, some is lost due to solubility in the product and some is lost when venting to reduce levels of impurities from the recycle. As needed, reformer off gas is added to the recycle stream to make up hydrogen losses. The recycle stream must be introduced into the reactor at a sufficient rate to insure that a minimum hydrogen partial pressure is maintained at the worst case. The minimum hydrogen partial pressure required is dependent on the catalyst used, and is usually in the range of from 100 to 250 psia.
In the prior art to Holcombe, a constant fresh feed flow was employed and hydrogen flow rates for the worst case provided more than enough hydrogen as the desorbed normals flow rate decreased toward the end of a desorption cycle. The system of Holcombe maintains the flow rates of hydrogen in the recycle and the combined reactor feed at constant levels, and varies the flow rate of fresh feed in an inverse relationship with the desorbed normals in the recycle. By thus eliminating fluctuations in hydrocarbon flow rates to the reactor, the recycle flow rate to the reactor is reduced without risking an insufficient partial pressure of hydrogen to protect the catalyst.
It would be desirable, however, to yet further reduce recycle flow rates and operate reactors and adsorbers at more favorable conditions and to otherwise improve operating efficiencies based on constant quantities of adsorbants and catalysts.